The family Acanthuridae with its six charismatic genera are some of the most beloved and recognisable fish both in the field and in the trade, serving as iconic reef denizens which play important roles in herbivory. The six genera holds about eighty or so species spanning a myriad of colours, sizes and forms, with certain larger species being of human importance in the form of food; eg. Naso spp. All species of Acanthurids regardless of genera are noted for their caudal peduncular spines, which may or may not be retractable depending on the species.
Naso and Prionurus are well noted for their often multiple unretractable spines, while Acanthurus, Ctenochaetus, Paracanthurus and Zebrasoma have only one. The presence of this unique morphological feature has earned this family the scientific name Acanthuridae, where “akantha” means thorn in greek. Colloquially they are aptly known as Scalpel Tangs, or Surgeonfish. The spines are very effective in sparring and combat, and can inflict painful cuts on intruding fish as well as careless humans. In this article we discuss the speciation and hybridisation of the A. nigricans complex of the genus Acanthurus.
The genus Acanthurus is the largest and most populous in the family, and contains within in many species groups with similar and closely related sister members. The nigricans complex is perhaps the most well known with four members, all of which are very popular in the aquarium trade and to a very negligible extent, commercial fishing. Unlike many distinct allopatric sisters, the nigricans complex is unusual with a few notable overlaps. While A. leucosternon, A. japonicus and A. achilles maintain disparate ranges, all three species are sympatric with the basal and very wide ranging A. nigricans.
Acanthurus nigricans has the most aggressive distribution of the group, being found in many locations across the Pacific, from the Ryukyus to the Great Barrier Reef, eastward to Hawaii as well as the French Polynesia. It’s distribution in the Indian Ocean is restricted only to the Cocos-Keeling and Christmas Islands as well as the Chagos archipelago. This species is remarkable for successfully crossing the Eastern Pacific Barrier, where it can be found in the Revillagigedo Islands, Cocos Island, Galapagos Islands, and the coast of Mexico.
A. nigricans is ash black over much of its body, fading to a lighter tone towards the rear. The base of its dorsal and anal fin are rimmed in yellow, which lends it its common name the Gold-rim Tang. The caudal peduncular spine is small, and sits within a groove where it is semi retractable. The spine is covered in a yellow sheath of the same hue as the dorsal and anal fin bases. A. nigricans has a white caudal fin that is sometimes slated in very light blue, and is decorated with a post-marginal yellow band. The outer edges of all its median fins are outlined in blue, and a white streak sits just under its eye which lends it its other common name, the White-cheek Tang.
With its huge range, A. nigricans is sympatric with all other members in this genus, and hybridises with every one of them. In the Ryukyus and the Philippines, A. nigricans overlaps with the very similar A. japonicus, where the two species hybridises. In the narrow strip of Bali where the two oceans mix, A. nigricans overlaps and hybridises with A. leucosternon to produce a rather beautiful hybrid. Eastwards in Hawaii and the Micronesian island chains, it hybridises with Acanthurus achilles.
In the very similar Acanthurus japonicus, the fish is matte grey with a yellowish-brown wash which lends it the name of Powder-brown Tang. It shares the same yellow rimmed features as the preceding species, but in A. japonicus the rims are very much more extensive and touches at the caudal peduncle, where it very often spreads posteriorly in a yellow suffusion toward the anterior half of the body. The dorsal fin has a bright orange streak on the rear portion, and all its median fins are outlined in blue.
Unlike A, nigricans, A. japonicus has a more extensive white cheek marking which spreads downwards to its upper lip. The caudal fin is similarly coloured in cloudy white, but it lacks a yellow post-marginal band. Instead, a short yellow streak sits just as the caudal fin base.
A. japonicus has the smallest range of the four species, being found only in Sulawesi, Japan and the Philippines. It is known to hybridise with A. nigricans, but because of the similarities in both species, potential hybrids are difficult to distinguish.
The rather similar coloration and markings between A. nigricans and A. japonicus means that any potential hybrids would look similar to either parents without any strong charismatic traits. This hybrid is therefore, the most elusive, but not because it doesn’t occur as readily as the others, but because separating it from either parents of pedigree lineage would require closer inspection. The above specimen is a pretty good example of a suspected hybrid, with traits of both A. nigricans and A. japonicus being expressed.
The specimen above differs from either parents by the following keys. Its cheek spot is more prominent than A. nigricans, but less extensive than A. japonicus. The caudal fin is also broadly suffused in yellow, versus a thin post-marginal band in pure blooded A. nigricans. Both dorsal and anal gold-rims are more extensive than A. nigricans, and spread further into the fin region, encoraching weakly also into the caudal peduncle area. Lastly, the anterior portion of the dorsal fin has an orange streak less prominent than A. japonicus, but more so than A. nigricans.
The traits are more or less intermediates of both species and shows up pretty well when closely inspected. A. japonicus is strictly pacific in distribution, but is confined to the Indo-West pacific.
Acanthurus achilles can be differentiated from the other members by a number of key distinct features. In this beautiful and highly prized species, the ground colour is a deep inky black. The caudal peduncle is decorated with a large orange tear drop pattern that only develops with age. In juveniles, only the peduncular spine sheath is orange. The dorsal and anal bases are rimmed in the same orange, but only slightly and does not spread as far out with the preceding two species. The tail is unique in being deeply emarginate, (doubly emarginate in large specimens) and is white with an orange post marginal band. The pelvic fins are white and its median fins are outlined in white instead of blue. The usual white cheek spot for the species group is represented by a streak on the preopercular region, and not below the eye.
Acanthurus achilles is wide ranging across the Western Pacific, but is mostly confined to Oceania and the various islands that pepper it. It is found in Hawaii, where it poses commercial value mainly for the aquarium trade and very negligibly for consumption. It is also known from from Wake, Marcus, and the Mariana islands. In the Eastern Central Pacific, it can be found in the southern tip of Baja California, Mexico where it is rare.
This species hybridises with A. nigricans in Hawaii and Micronesia to form a rather beautiful hybrid. In the hybrid between A. achilles and A. nigricans, very little traits of the former are expressed. The hybrid is coal black overall and lacks the cheek streak of A. nigricans. The orange teardrop feature that is so strongly associated with one of the parent species is very hardly seen, and the rear half is strongly characteristic of A. nigricans.
The strongest trait that A. achilles imparts to its hybrid is the orange tail, which manifests very nicely with the yellow in A. nigricans to form a bicolored suffusion. This hybrid is rare and is found mostly where A. achilles is more prevalent than A. nigricans, and is highly prized in the aquarium trade.
The final member of this complex is the unmistakable Acanthurus leucosternon, very aptly named the Powder Blue Tang. This species is predominantly a specialist of the Indian Ocean, where it can be found in Africa, eastwards to the Maldives and Andaman Sea. The Southwestern portion of Indonesia is the eastern most limit to its Indian Ocean range, and it strays very weakly into the Pacific side of Indonesia such as Bali, where species from both oceans mixes.
A. leucosternon is a beautiful powder blue with a jet black mask. The white cheek spot is very weakly presented below the eye, if at all, and the species has a white collar around its gills and pectoral fin. The species does not possess any of the usual dorsal and anal fin rim markings that the three other Pacific species share, but instead has a solid yellow dorsal fin which reaches only up to the caudal peduncle. Its ventral, anal and caudal fins are entirely white, and the caudal is outlined on all edges in black.
It comes with the least surprise that this species bears the least resemblance to any of the other members of this complex. Being found in the Indian Ocean, A. leucosternon has evolved on its own away from the other members by the separation of two oceans. Although all members of the nigricans complex can be found in groups, it is only with A. leucosternon that this “grouping” behaviour can be extrapolated into large flocks bearing hundreds or even thousands of individuals.
Acanthurus triostegus is another species that is capable of swarming, and it often mixes with A. leucosternon to form huge roving plagues that strip the reef clean of filamentous algae. The swarming phenomena is unusual, and like terrestrial locusts, is triggered by very specific cues. In areas where algae gardens are defended fiercely by other Acanthurids for example, swarming helps to collectively refute rivals for the common goal of feeding. Under normal circumstances though, they are often solitary or in pairs and can be aggressive towards intruders.
This species hybridises with the promiscuous A. nigricans as well in a very narrow strip around Bali and the Christmas Islands.
Unlike the hybrids of A. achilles x A. nigricans, A. leucosternon x A. nigricans is very variable with a few intermediate designs. The hybrids can either show traits more strongly skewed towards A. leucosternon, or those leaning more towards A. nigricans. This spectrum of intermediate hybrid forms poses the question as to whether or not this hybrid is sexually viable, and if it is capable of backcrossing with either parents to form hybrids that are more diluted in the favour of either species.
The specimen above shows traits more strongly expressed by A. leucosternon, but is still obviously infused with characteristics of A. nigricans. The darkening of the ventral and anal fins, as well as the presence of a diluted yellow post-marginal caudal band are all characteristics of the latter. The white collar of A. leucosternon is also weakly expressed, and so is the cheek spot of A. nigricans.
The photo above shows a hybrid individual with very strong traits of A. nigricans. So strong in fact, that the only noticable traits of A. leucosternon are the deep navy blue suffusion on the body as well as the weakly expressed collar. The median fins and tail are almost entirely A. nigricans looking, especially the clearly defined gold rims. This photo is also curious in that it shows the hybrid courting with a pedigree A. nigricans, which suggests that the hybrid does engage in spawning activity. Whether the hybrid is viable or not is a different question, but the spectrum of intermediates and courting with pedigree parents seems to suggests that it is.
Joe Yaiullo of the Long Island Aquarium has also documented videos of this hybrid engaging in pre-spawning rituals of pedigree A. leucosternon.
Outside of this complex, A. leucosternon is known to hybridise with Acanthurus bariene, but this hybrid appears to be very rare and most likely accidental. The similarities as well as frequency of hybridisation within the nigricans species complex suggests the closely related nature of the group. Three of the species, namely A. leucosternon, A. japonicus and A. achilles maintain allopatric distributions, while A. nigricans touches base with all three with its large range occupation. It is however only narrowly parapatric with A. leucosternon in Bali and the Christmas Islands. A. nigricans forms hybrids with all three members of the group where they overlap.